This invention relates to a circuit for determining when a voltage falls between preselected upper and lower voltage limits.
Many applications, such as in voltage controlled processes, instrumentation and checkout (or verification) functions, sometimes require an accurate means for determining when the control voltage is within a preselected range of voltage levels, as opposed to the more simple case of determining if the voltage is above or below a single preselected level. The circuit for the latter is commonly referred to simply as a "comparator," while for the former, more complex case, it is commonly called a "window comparator." A typical application for a window comparator is to check the feedback signal (V.sub.FB) of a servomechanism to determine that it has been positioned to a preselected position (V) within a tolerance .+-.(.DELTA.V/2).
A window comparator transmits an output voltage or current signal at a predetermined level only when the input voltage is between two selected levels. At all other times the voltage or current signal transmitted by the window comparator is at another level. A simple check of the output signal level with then indicate the state of the input signal vis-a-vis the selected set point V.+-..DELTA.V/2, where .DELTA.V is the selected window width. For that reason a window comparator is sometimes said to have a "go-no-go" range, while a simple comparator is said to have a "go-no-go" point.
In the past, a window comparator has been implemented with two simple comparators, one biased at the lower limit level and one biased at the upper limit level. The outputs of the two simple comparators are then connected to provide a wired OR output that is of a first level whenever the range is exceeded, and of a second level whenever the range is not exceeded. A problem with this parallel-comparator approach is the difficulty in maintaining stable reference levels, particularly when a narrow window is involved. If the two references should drift in opposite directions, the window becomes too large, or too small. In the case of both drifting toward each other, there may be no window left at the time the comparator output is checked for a go-no-go decision. Such independent drifting of the reference levels occurs due to changes in ambient temperature, aging of components and other environmental factors. The hysteresis of the simple comparators themselves will also affect the window size.
For the application which requires a narrow window, e.g. 10 millivolts, within a possible 10 volt range, the upper and or lower limits of the window must be maintained to within one part in 10,000 to maintain the window width to within 10% of the present value. The same window stability may be obtained by the present invention in which the input signal is first compared with a set point voltage, V, to determine when the input signal first enters the window range, V + .DELTA.V, and then comparing the excess of the input signal over the set point level with the window, .DELTA.V. This permits setting the window, .DELTA.V, to within just one part in ten, instead of to within one part in 10,000
A tandem window comparator Model 4021/25 has been commercially available from Burr-Brown Research Corporation, but it is comprised of two operational amplifiers in a circuit which provides an output signal proportional to the absolute value of difference between the input signal and a reference voltage. The output of the absolute value circuit is then compared with the window voltage, .DELTA.V. While such a tandem arrangement of an absolute value circuit with a simple comparator may permit setting the window to within one part in less than 10,000, or even to within one part in ten, it requires three operational amplifiers, and four diodes: two operational amplifiers and two rectifying diodes for the absolute value circuit, and one operational amplifier and two clamping diodes for the window comparator. The present invention requires only two operational amplifiers and two switching diodes. Moreover, it does not allow independent setting of the reference voltage as the present invention does. This is an important difference since adjustment of the reference voltage in the Burr-Brown circuit varies the window level and window width. This makes settings more difficult and complex to make, particularly for remote operation.